mgcp_protocol: Avoid code duplication between virtual + other trunks

There were two code paths that were supposed to do exactly the same,
but then in Change-Id I3994af016fb96427263edbba05f560743f85fdd4 only
one of the two was modified, resulting in OS#4034

Let's
* dynamically allocate the virtual trunk
* rename mgcp_config.trunk to mgcp_config.virt_trunk to clarify
* as a result, abolish copy+pasted code for trunk initialization

Change-Id: I54762af6d417b849a24b6e71b6c5c996a5cb3fa6
Related: OS#4034
8 files changed